[css-color-5] Actually allow percentages for adjusters

Per previous grammar, they were basically argument-less keywords

Author: LeaVerou

css-color-5/Overview.bs

@@ -100,10 +100,10 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 </pre>
 
 <pre class='prod'>
-	<dfn>&lt;color-adjuster></dfn> = [
+	<dfn>&lt;color-adjuster></dfn> = [ [
 			  <<srgb-adjuster>> | <<hsl-adjuster>> | <<hwb-adjuster>>
 			| <<xyz-adjuster>> | <<lab-adjuster>> | <<lch-adjuster>>
-		] || alpha
+		] | alpha ] <<percentage>>?
 	<dfn>&lt;srgb-adjuster></dfn> = red || green || blue
 	<dfn>&lt;hsl-adjuster></dfn> = <<hue-adjuster>> || saturation || lightness
 	<dfn>&lt;hwb-adjuster></dfn> = <<hue-adjuster>> ||  whiteness || blackness
@@ -133,9 +133,9 @@ lightness means the LCH Lightness, which is the same as Lab lightness,
 but different to the HSL Lightness.
 
 Only the [=color-adjuster=]s defined for a given [=colorspace=] are available.
-The alpha adjuster is available regardless of color space.
 For example,
 it is not possible to use the HWB [=whiteness=] adjuster unless the colorspace is [=hwb=].
+The alpha adjuster is available on any color space.
 
 If the specified colorspace has a smaller gamut
 than the one in which the color to be adjusted is specified,